Photopolymers are starting to find widespread application in the preparation of printed circuit boards and other electronic devices. These materials are applied to copper-laden circuit boards by screen printing and/or stenciling techniques which are well known in the art.
Applicant's prior U.S. Pat. No. 4,409,903 describes a semi-automatic device that uses a horizontal print stroke to print a photoimagable ink on one side of a board. To print both sides of a circuit board, the screened coating must first be cured by passing the board through an oven, then the board is rescreened on the reverse side. Many individual handling steps are required: screen the first side, hold, bake, cool, hold, screen the second side, hold, bake, cool, hold, and then expose. Accurate baking of photopolymer ink is critical to the success of the process. If the boards are screened in two separate operations, they must be baked after each screening, with the result that one side will be baked twice. The latitude of many photopolymers useful as inks is limited so that one side must be slightly under-cured while the other is slightly over-cured. In addition, copper on the uncoated side of the board is unprotected and tends to oxidize during the first baking step. Another drawback is that holes in circuit boards readily clog with ink, requiring much harsher developing methods to clear the hole. Finally, since the first inked side is slightly under-cured, and may be soft and tacky when the second side is inked, the surface of the first inked side can be susceptible to contamination and easily damaged.
Conventional screening mechanisms not only require multiple handling steps to screen both sides of the circuit board, but the sequence of steps severely limits the feasibility and extent of automation.
U.S. Pat. No. 4,275,655 issued to Gerrard P. Artaud et al describes a device for simultaneously printing both sides of a target. More particularly, the '655 patent describes a device for acid resist printing an identical image on both sides of a continuous metallic web that is drawn through the printing zone under tension. Opposing squeegees are mounted on a unitary carriage to ensure mirror image printing. There is little disclosure in this patent that is useful in developing apparatus to facilitate the manipulation of circuit boards and chases in various stages of closure, flooding, printing and opening.